1. Field
The present disclosure is widely applicable to electronic integrated circuits (“ICs”).
2. Related Art
It is usually desirable for an IC to operate from a single voltage supply. However, many ICs require two or more different voltage supplies, for example to provide internal bias supplies, for ideal operation. Such different supplies can be provided externally to the integrated circuit, but this is undesirable from a user standpoint. Providing additional supplies is not only inconvenient for the user, but may also cause the conductors coupling such external supplies to the IC to be unduly long, which among other difficulties may cause undesired emissions if noise is present on the supply. As such, it is a common practice to provide auxiliary circuitry on ICs to generate such additional bias generation voltages, or other voltage supplies, as may be required for circuit operation needed. Charge pumps are one of the most common of such auxiliary voltage-generating circuits used in ICs.
However, charge pumps have characteristics that have rendered them difficult to use in certain applications. In particular, charge pumps have invariably created a substantial amount of electrical noise. Regulations have been promulgated to prevent electronic devices from interfering with each other, and such regulations establish maximums for allowable emissions. In some applications the noise generated by a charge pump may cause the IC or system in which such IC is disposed to exceed such maximum permitted noise emissions.
For example, most radios, cell phones, TVs, and related equipment today require an “RF switch” to control connections between various transmitter and receiver circuits (“RF” is used generically herein to mean any reasonably high frequency ac signal). At least one auxiliary voltage generator is often needed to satisfactorily bias the FETs that comprise a semiconductor RF switch. Many of the products that employ RF switches are transceivers, such as cell phones, that are subject to stringent regulatory limitations on the electrical signals that it is permitted to emit. Because such RF switches are directly connected to the transceiver antenna, even very small amplitude noise signals generated by the bias generator of the RF switch will be all too efficiently radiated. It has been found that the noise generated by a conventional charge pump may be sufficient to cause a cell phone employing an RF switch using such charge pump to exceed the maximum noise emissions permitted by applicable regulations. As such, a noisy charge pump can render such a cell phone unsuitable for its commercial purpose.
Consequently, bias generation circuits, such as charge pumps that generate far less noise than conventional charge pumps are crucial for certain applications. Low noise bias generation circuits will find advantageous employment in a wide range of integrated circuits, whether to satisfy regulatory spurious emission limits, or to avoid interference with other local circuitry. Such circuits must also be efficient in terms of integrated circuit area consumed, and, especially for battery operated devices such as cellular telephones, must be efficient in terms of power consumption.
Additionally, it is often useful to control an output voltage of a charge pump by means of a feedback control loop that includes a differential-input operational transconductance amplifier (“OTA”). OTA output common mode voltages include the effects of various offsets, including input signal misalignment, differential input offset voltages, finite gain of input common mode signals, and other mismatches that may occur throughout the OTA. Nulling the effect of such offsets is particularly useful for amplifying small signals. Adjusting output voltage levels is also useful for permitting maximum gain before the signal is clipped.
The method and apparatus presented herein address the need for low-noise, high efficiency bias generation circuits, including charge pumps, regulation control and amplification circuits, bias level setting circuits and, particularly for the capacitive coupling of low-noise clocking waveforms, efficient active bias circuits. Various aspects of the bias generation method and apparatus described herein will be seen to provide further advantages, as well.